1. Field of the Invention
The present invention relates to a reflection-type projector, and more particularly, to a reflection-type projector having an improved light separation structure in order to reduce the effect of light reflected from a window surface of an image forming means, such as a micromirror device.
2. Description of the Related Art
In general, in a reflection-type projector employing a micro-mirror device, a plurality of micro-mirrors provided to correspond to pixels of a screen are independently operated, and the paths of rays reflected from the movable mirrors are differentiated by the movable mirrors, so that an image can be formed.
Referring to FIG. 1, a reflection-type projector employing a conventional micromirror device includes a light source 10 emitting light rays, a color wheel 20 for embodying a color image, a glass rod 30 for generating uniform rays, a lens element 40 for converging incident rays, a light path changing means 50 for changing the paths of light rays, a micromirror device 60 for forming an image, and a projection lens unit 70 for projecting incident rays onto a screen to form a magnified image on the screen.
The light path changing means 50 is provided with a beam separation prism 51 which changes the paths of rays by transmitting the incident rays entering the beam separation prism 51 at angles smaller than a critical angle and by reflecting the incident rays entering the beam separation prism 51 at angles greater than a critical angle. A compensating prism 55 is further provided to compensate for deviations of the paths of the separated beams.
The beam separation prism 51 has an incident surface 51a through which the rays emitted from the light source 10 enter, a critical surface 51b slanted with respect to the incident surface 51a, and an entrance-exit surface 51c facing the micro-mirror device 60 and transmitting incident rays reflected from the critical surface 51b and rays reflected from the micro-mirror device 60. The critical surface 51b reflects the incident rays entering the beam separation prism 51 via the incident surface 51a at angles greater than the critical angle toward the entrance-exit surface 51c, and transmits the incident rays from the micro-mirror device 60 at angles smaller than the critical angle. The compensating prism 55 is disposed so that one of its surfaces faces the critical surface 51b of the beam separation prism 51 and compensates for the deviations of the paths of the rays traveling toward the projection lens unit 70.
As shown in FIGS. 2 and 3, the micro-mirror device 60 comprises a plurality of movable mirrors 62 installed on a substrate 61 to correspond to respective pixels, a plurality of posts 63 each for supporting the respective movable mirrors 62 so that the movable mirrors 62 can pivot, and a window 65 for protecting the movable mirrors 62. Each movable mirror 62 is driven to rotate by electrostatic attraction forces between the movable mirror 62 and electrodes provided on the substrate 61.
Therefore, the angles of the reflection surfaces of the movable mirrors 62 are individually selected according to whether the electrodes corresponding to the respective pixels are driven or not, that is, depending on whether the electrodes are in an on-state or in an off-state. Therefore, an image is formed by causing only necessary rays reflected from mirrors 62 in the on-state to be directed to the projection lens unit 70, and by causing unnecessary rays reflected from mirrors 62 in the off-state to travel away from the projection lens unit 70 so as not to be used.
In the micro-mirror device 60, the incident rays are reflected from the surface of the window 65 as well as the movable mirrors 62. The rays reflected from the surface of the window 65 are called surface-reflected rays.
Reviewing FIG. 1, a reflection-type projector employing a conventional micro-mirror device is configured so that both the necessary reflected rays and the surface-reflected rays reflected from the micro-mirror device 60 travel after passing through the critical surface 51b, and a portion of the surface-reflected rays are directed to a screen together with the necessary rays after passing through the projection lens unit 70. Therefore, the reflection-type projector employing a conventional micro-mirror device is problematic, in that the contrast of an image formed on the screen deteriorates noticeably.
To solve the above problem, it is an objective of the present invention to provide a reflection-type projector that has an improved light separation structure, so that surface-reflected rays reflected from the surface of a window of an image forming means may not be directed to a screen.
Accordingly, to achieve the above objective, there is provided a reflection-type projector including a light source for generating and emitting light; an optical path changer for changing the traveling path of an incident beam; an image former that is provided to correspond to respective pixels for reflecting incident rays to form necessary reflected rays and unnecessary reflected rays having reflection paths different from each other according to the driven states of the image former and which has a window for enclosing the image former; and a projection lens unit for magnifying incident rays and projecting the rays, wherein the optical path changer is a beam separation prism having: a first critical surface that is slanted so that incident rays from the light source pass through the first critical surface, and the necessary reflected rays and surface-reflected rays reflected from the window are reflected by the first critical surface; an entrance-exit surface that faces the image former and one surface of the projection lens unit and through which rays enter or exit; and a second critical surface slanted with respect to the optical axis of necessary reflected rays reflected from the first critical surface in order to transmit unnecessary reflected rays and surfacereflected rays reflected from the first critical surface, and in order to reflect necessary reflected rays reflected from the first critical surface to the projection lens unit